Generally, an air-conditioning installation for a motor vehicle comprises valves including shutters, the opening and closing of which are motor-controlled by means of electric motors, such as stepper motors with permanent magnets, each associated with a reduction unit.
These air-conditioning installations for motor vehicles, which can be of the type as disclosed in DE 4343385 and FR 2731852, have many drawbacks. Besides the problems of manufacturing costs related to the power used and the size of the motors used, the movements of the shutters, when opening and closing, as well as the motors themselves generate sound problems that can difficultly be coped with through sound-insulating means, since the sound waves are conveyed through the ventilation conduits.
The noise generated by the shutters and the motors is mainly due to the characteristics of said motors, which will be set forth hereinafter by means of the description of a configuration of an air-conditioning valve-actuator for a motor vehicle, this configuration being the most often used one.
For reasons of simplicity and manufacturing costs for the control electronics, the control of this air conditioning valve actuator occurs at a fixed frequency of about 200 Hz The stepper motor includes a rotor with 6 pairs of poles, which can thus adopt 24 different positions, or steps, per revolution. Now, in a stepper motor, to each current-supply impulse corresponds a constant elementary rotation by one step, so that a determined number of impulses results into a corresponding number of steps and, hence, into a known rotation of the rotor. Therefore, the angular distance covered by the rotor between 2 steps is 15°, so that a control at a fixed frequency of 200 Hz results into a rotor speed of 3000°/second.
In order for this speed to be usable, the motor is associated with a reduction unit the gear train of which allows increasing the outlet torque and reducing the amplitude of the angular pitch. By using a reduction unit with a ratio of about 300, the fixed speed of rotation of the air-conditioning shutter is about 10°/second.
This being said, the need for an operation according to a so called “start-stop” mode, i.e. for instantaneously switching over, between two successive steps of the motor, in its starting phase, from 0 to 200 Hz, requires dimensioning the motor so that it be capable of accelerating the shutter inertia of said air-conditioning shutter and the rotor's own inertia within a time period close to 5 milliseconds, which corresponds to the duration of one step.
Now, in a “start-stop” operating mode, the torque of the motor is clearly lower than that of the same motor operating in the dynamic mode, since in the latter there is no need for an instantaneous acceleration of the inertia of the rotor and that of the air-conditioning shutter.
Therefore, in order to reach, at the outlet of the reduction unit, the torque required for moving an air-conditioning shutter in a “start-stop”-type operation, it is necessary to oversize the motor.
Furthermore, with a coil resistance maintained constant, the available mechanical power, the torque and the electric-power consumption extremely quickly increase with the power-supply voltage. Now, the air-conditioning valve has been designed so as to carry out its mission also at a degraded 8-Volt battery voltage, i.e. the torque required for moving the air conditioning shutter should also be reached at an 8-Volt voltage. The voltage of a non-controlled battery however varies between 8 and 14 Volt, so that the motor mostly operates at a voltage higher than 8 Volt, which, because of the increase in available mechanical power, torque and electric-power consumption, besides the increased noise, is also prejudicial to the valve's lifetime, since this causes, on the one hand, at the level of the reduction unit, an excessive blocking torque likely to damage the toothing of the latter and, on the other hand, an overheating of the winding of the motor.
Furthermore, in the field of the air-conditioning valve-actuators, the use of stepper motors requires, at each start, a re-initialization achieved by bringing the shutter of the valve into abutment. In the “start-stop” operating mode at 200 Hz, the valves generate a characteristic noise when the valve arrives in abutment, since the stepper motor is still at its synchronism speed and the rotor starts vibrating at about the stop position. Since the reduction unit is slightly elastic, the motor even accumulates energy while setting this type of spring formed by the gears and, in some applications, one can even see that the motor turns back by some tens of steps, under the action of this elasticity, when the current supply has been interrupted in the windings.